Field of the Invention
The present invention relates to a fringe field switching (FFS) liquid crystal display panel and a liquid crystal display that includes the liquid crystal display panel.
Description of the Background Art
In recent times, many new displays that include a thin and flat display panel using the principle of liquid crystals or electroluminescence instead of conventional cathode-ray tubes have been used. Liquid crystal displays typified by the new displays are not only thin and light but also capable of being driven at low voltage.
The liquid crystal display device includes two substrates with a liquid crystal layer sealed therebetween. One of the substrates is an array substrate in which a plurality of pixels arranged in matrix form a display region and the other of the substrates is a counter substrate. The counter substrate may be referred to as a color filter substrate particularly in a case in which a color filter is formed on the counter substrate.
A thin-film transistor (TFT) liquid crystal display particularly enables display of high image quality in which crosstalk is less likely to occur because each of pixels on the substrate includes a TFT serving as a switching element and is capable of independently maintaining a voltage for driving the liquid crystal layer. Each of the pixels includes gate lines (scan lines) for controlling ON and OFF of the TFT and source lines (signal lines) for input of image data. A region surrounded by the gate lines and the source lines usually corresponds to each of the pixels.
Fringe field switching (FFS) having excellent characteristics of a viewing angle and high light transmittance has been proposed for recent liquid crystal displays (see Japanese Patent Application Laid-Open No. 2003-84303 and Japanese Patent Application Laid-Open No. 2009-133954). The FFS liquid crystal display performs display by applying a fringe electric field (oblique electric field including both components of a lateral electric field and a vertical electric field) on the liquid crystal layer. The FFS liquid crystal display includes a transparent pixel electrode and a transparent common electrode on one side of the array substrate, the transparent pixel electrode and the transparent common electrode being placed on top of each other with an insulating film therebetween. The lower-layer side is usually a plate-shaped electrode (may have a shape of a plurality of branches) while the upper-layer side is usually an electrode that is located in almost the same position as the plate-shaped lower-layer side and has a plurality of gaps serving as slits to control liquid crystals by an electric field from the side of the lower electrode through the slits. At this time, both of the pixel electrode and the common electrode can achieve high transmittance by being formed of a transparent conductive film.
Such an FFS liquid crystal display having a wide viewing angle and high transmittance has been developed for various applications. Thus, as a request placing importance on design of products, demand for a narrow frame having a smaller width around the display region has been growing.
The array substrate that includes the plurality of pixels arranged in matrix to form the display region in the liquid crystal display typically includes a dummy pixel region located around the display region, dummy pixels that imitate display pixels in the display region being arranged in the dummy pixel region (see Japanese Patent Application Laid-Open No. 1-293317 (1989)). A display portion of the display sometimes refers to a region in which the display pixels and the dummy pixels are arranged on the array substrate, but strictly speaking, the display region and the dummy pixel region are different. The reason is that there is difference between the display pixels that contribute to display with light passing therethrough and the dummy pixels that do not directly contribute to display. Thus, a light-shielding material such as a black matrix may be located on the color filter substrate facing the dummy pixels, or the dummy pixels and the display pixels may vary in conditions of driving the TFT in some cases.
The dummy pixel is manufactured so as to have the same size and the same structure as those of the display pixel in the display region in terms of appearance. Nevertheless, a structure in which a dummy pixel having a width measurement smaller than that of a display pixel has been proposed to meet the demand to narrow the width of the frame as described above (see Japanese Patent Application Laid-Open No. 9-5780 (1997)).
However, according to Japanese Patent Application Laid-Open No. 9-5780 (1997), the dummy pixel reduced in size changes a storage capacitance and a capacitance and a resistance of lines in the dummy pixel in comparison with those in the display pixel in the display region. Consequently, the dummy pixel and the pixel in the display region discontinuously vary in electrical or optical characteristics. Thus, a situation in which image quality is likely to discontinuously vary occurs in the vicinity of the dummy pixel, resulting in failures that lower display quality, such as peripheral unevenness in the display region of the liquid crystal display. Further, the reduced storage capacitance formed in the dummy pixel increases variations in potential of signal lines due to static electricity, and thus a frequent occurrence of line failures due to a short circuit between the gate lines and the source lines in the display region decreases yields.
Particularly in the FFS liquid crystal display, the fringe electric field (oblique electric field including both components of the lateral electric field and the vertical electric field) is applied to the liquid crystal layer for display. Therefore, the FFS liquid crystal display tends to be affected by variations in electric field around the display region, and the failures and the decrease in yields also have greater influences.